1. Field of the Invention
The present invention relates to an exposure mask for use in lithography in the manufacture of semiconductor devices, and a method and an apparatus for manufacturing the exposure mask.
2. Description of the Related Art
The degrees of integration and miniaturization of semiconductor integrated circuits have increased steadily, and shortening the wavelength of an exposure light source has been studied to meet this requirement. Recently, on the other hand, a phase-shifting method of improving an exposure mask without changing an exposure light source has attracted attention. This phase-shifting method aims at improving pattern accuracy by forming a portion called a phase shifter for inverting a phase and thereby removing the influence of positive interference of light between adjacent patterns. Although a variety of phase-shifting methods have been proposed so far, a Levenson method, among other phase-shifting methods, is particularly known well as a method of dramatically improving resolving power and focal depth.
The Levenson method produces a negative interference by setting a 180.times. phase difference between light components transmitted through adjacent light-transmitting portions. The Levenson method has a large effect of improving resolving power for periodic patterns such as line-and-space patterns. If, however, the phase difference of 180.degree. is set in situations where three or more patterns are formed adjacent to one another, phases become equal at least at one portion.
Consequently, no resolving power improving effect can be obtained in this portion in which the phase difference between adjacent portions becomes 0. It is therefore required to improve the design or the like in order to put this method into practice for actual device patterns.
A halftone method is one phase-shifting method requiring no change in device design. This method uses a translucent film, instead of a light-shielding film, and sets a 180.degree. phase difference between light transmitted through the translucent film and light transmitted through a transparent portion, thereby reducing the interference of light which causes a decrease in pattern resolution. To take advantage of the full phase-shift improving effect of the halftone method, it is necessary to optimize the transmittance of the translucent film and the phase difference between the light components transmitted through the translucent film and the transparent portion.
One example of this half-one method is the method disclosed in Jpn. Pat. Appln. KOKAI Publication No. 4-136854 in which the phase and the transmittance are adjusted by using a multilayered film. However, the use of the multilayered film requires two transfer steps. In addition, if defects occur in the lower layer, it is difficult to correct these defects. Adjusting the phase and the transmittance by using a single-layer film also has been examined as disclosed in Japanese Patent Application No. 4-327623. However, to simultaneously control the transmittance and the phase difference by a single-layer translucent film, the composition of a film to be used as the translucent film is limited.
Generally, a film consisting of a material having an intermediate composition, i.e., a material containing a semibonded state is used as the translucent film. Therefore, variations in the physical properties of the translucent film may readily take place upon irradiation with light during exposure. Since the irradiation direction of exposure light is from a transparent substrate to the translucent film, the influence of the photoirradiation reaches a maximum in the interface between the translucent film and the transparent substrate. The translucent film has a large absorbance to an exposure wavelength. Therefore, a thermal reaction occurs due to an optical reaction taking place near the interface or due to heat produced by this optical reaction. This results in loss of a resistance against the irradiation of exposure light if a measure of only stabilization of the film quality on the surface of the translucent film is taken by an oxidation treatment. Consequently, the physical properties of the translucent film vary primarily near the interface with the transparent substrate.
With this reaction, the phase difference and the transmittance of the translucent film of a halftone phase-shifting mask are shifted from their respective desired values upon irradiation with light during exposure. After the mask is manufactured, therefore, the shape of the transferred resist pattern is degraded or the focal depth is decreased due to the deterioration with time of the amplitude transmittance and the phase difference.
Note that it is also possible to leave the translucent film to stand until physical property variations no longer take place. This method, however, is impractical because several years are required before the film quality is stabilized.
The present invention provides a method of solving the above conventional problems by using at least one of irradiation, heating, and oxidation as will be described later. As disclosed in Jpn. Pat. Appln. KOKAI Publication No. 3-131027, however, the end point of irradiation has been indirectly determined in conventional methods. That is, the relationships between the film thickness of an insulating film formed by irradiation and the irradiation temperature and between the film quality and the irradiation time are obtained beforehand, and the irradiation time is calculated by monitoring the temperature during the irradiation. Alternatively, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 3-278524, a method of compensating for the lamp output by measuring the transmittance and the reflectance to infrared light prior to irradiation has been performed. These methods, however, cannot directly measure the film thickness or the physical properties of a film being treated. Consequently, variations are produced in the physical properties of the film obtained after the treatment.
In conventional halftone phase-shifting masks as discussed above, the physical properties of a translucent film vary due to photoirradiation during exposure or with time, shifting the phase difference and the transmittance of the translucent film from their respective desired values. This leads to degradation in the shape of a transferred resist pattern or decrease in the focal depth. In addition, exposure masks capable of achieving the maximum phase-shifting effect are difficult to manufacture with a high reproducibility, since it is not possible to directly measure the transmittance and the phase shift during the irradiation or heating mentioned above.